Belt driven accessory systems are in common use to transfer power from an internal combustion engine crankshaft to accessory components typically including an alternator (generator), water pump, oil pump (power steering), air conditioning compressor (via electromechanical clutch). These components are usually fixed position mounted and utilize an automatic belt tensioner to provide constant belt tension and take-up of belt slack.
Internal combustion engines generate rotating power at the crankshaft only when a combustion event occurs. This, in effect, is a pulsed system whereby the closer the spacing of the combustion events, the smoother the rotational consistency of the engine. For each combustion stroke, the crankshaft will exhibit acceleration, then deceleration, until the next combustion stroke. In general, the slower the rotation of the engine, and the lesser number of cylinders (combustion events per revolution of crankshaft) tends to increase the magnitude of the pulsing effect.
In terms of the serpentine belt system, the crankshaft pulses are transferred to the belt as fluctuations in velocity. The velocity fluctuations of the engine are thus transferred to all the driven components in the system. Dynamic belt tension fluctuation is generated by the velocity fluctuation. Without considering the dynamic loading of the accessory components and consequent tension effects, it is obvious that the driven inertias will generate dynamic tensions as the belt continuously attempts to accelerate and decelerate such components.
Where the engine is smaller, four or five cylinders, and in the lowest speed ranges (idling area) the dynamic tension fluctuation is at the highest magnitude. The magnitude can be further increased by technological differences that serve to lower the engine rotating inertia (dual mass fly wheel) or increase the instantaneous acceleration (diesel, higher compression, etc.). Operating conditions can also have a significant effect, for instance, “lugging” where the engine is running below its ideal minimum speed (idle) at high power levels that attempt to increase the speed back to idle.
Under these circumstances, the dynamic belt loading can be so great that the belt tensioner cannot accommodate all the dynamic fluctuations. The results can include belt noise, belt slippage and forced vibration of the belt, tensioner and accessory components. Ultimately, durability is compromised.
It is possible to resolve this problem using a torsional isolator at the crankshaft, provided it has low stiffness. Such conventional torsional isolators have been employed for many years, but are bulky, costly, heavy, and display limited effectiveness. This limited effectiveness is generally the result of the drive having to be designed to carry the full power capability of the system, while rarely ever requiring the same. Thus, torsional isolators are typically too stiff.
Each of the noted functions has different engineering requirements for optimizing the system. For example, the resilient coupling function would optimally have a greater spring rate (a stiffer spring) than the spring rate utilized for performing the coupling/decoupling function. Optimally, a higher spring rate is desirable for transmitting driven rotational movement of the alternator pulley to the hub structure in order to accommodate the relatively high torsional forces, while a lower spring rate is desirable for the de-coupling function so that less force is exerted and thus less frictional wear and heat is generated by the mechanism during the de-coupling or overrun condition. Increasing the spring rate of the mechanism to accommodate the torsion transmitting function would be to the detriment of the coupling/de-coupling function, while decreasing the spring rate to accommodate the coupling/de-coupling function would be to the detriment of the torsion transmitting function.
Representative of the art is U.S. Pat. No. 6,083,130 which discloses a serpentine belt drive system for an automotive vehicle comprising a drive assembly including an internal combustion engine having an output shaft with a driving pulley thereon rotatable about a driving pulley axis. A sequence of driven assemblies each has a driven pulley rotatable about an axis parallel with the driving pulley axis and a serpentine belt mounted in cooperating relation with the driving pulley and with the driven pulleys in a sequence which corresponds with the sequence of the driven assemblies when related to the direction of movement of the belt to cause said driven pulleys to rotate in response to the rotation of the driving pulley. The sequence of driven assemblies includes an alternator assembly including an alternator shaft mounted for rotation about a shaft axis. A hub structure is fixedly carried by the alternator shaft for rotation therewith about the shaft axis. A spring and one-way clutch mechanism couples the alternator pulley with the hub structure. The spring and one-way clutch mechanism comprises a resilient spring member separately formed from and connected in series with a one-way clutch member. The resilient spring member is constructed and arranged to transmit the driven rotational movements of the alternator pulley by the serpentine belt to the hub structure such that the alternator shaft is rotated in the same direction as the alternator pulley while being capable of instantaneous relative resilient movements in opposite directions with respect to the alternator pulley during the driven rotational movement thereof. The one-way clutch member is constructed and arranged to allow the hub structure and hence the alternator shaft to rotate at a speed in excess of the rotational speed of the alternator pulley when the speed of the engine output shaft is decelerated to an extent sufficient to establish the torque between the alternator pulley and the hub structure at a predetermined negative level.
What is needed is an alternator isolating decoupler having a first wrap spring and a second wrap spring in parallel, each in turn arranged in series with a torsion spring and each wrap spring being releasably engaged with a one way clutch and releasably engaged with an alternator rotor. The present invention meets this need.